Regional cerebral phosphorus-31 magnetic resonance spectroscopy (31P-MRS) was performed in 10 non- demented Parkinson's disease patients and nine age-matched control subjects. Five of the patients undergoing 31P-MRS and four additional Parkinson's disease patients had cerebral 2-[18F]fluoro-2-deoxy-D-glucose PET (18FDG-PET), the results of which were compared with those of eight age-matched control subjects. All Parkinson's disease patients underwent neuropsychological testing including performance and verbal subtests of the Wechsler Adult Intelligence Scale—Revised, Boston Naming Test, Controlled Oral Word Association test (FAS Test) and California Learning Test to exclude clinical dementia. 31P MR spectra from right and left temporo-parietal cortex, occipital cortex and a central voxel incorporating basal ganglia and brainstem were obtained. 31P MR peak area ratios of signals from phosphomonoesters (PMEs), inorganic phosphate (Pi), phosphodiesters (PDEs), α-ATP, γ-ATP and phosphocreatine (PCr) relative to β-ATP were measured. Relative percentage peak areas of PMEs, Pi, PDEs, PCr, and α-, β- and γ-ATP signals were also measured with respect to the total 31P-MRS signal. Significant bilateral increases in the Pi/β-ATP ratio were found in temporoparietal cortex (P = 0.002 right and P = 0.014 left cortex) for the non-demented Parkinson's disease patients compared with controls. In the right temporoparietal cortex, there was also a significant increase in the mean relative percentage Pi (P = 0.001). 18FDG-PET revealed absolute bilateral reductions in glucose metabolism after partial volume effect correction in posterior parietal and temporal cortical grey matter (P < 0.01 and P < 0.05, respectively) for the Parkinson's disease group, using both volume of interest analysis and statistical parametric mapping. There were significant correlations between right temporoparietal Pi/β-ATP ratios and estimated reductions in performance IQ (r = 0.96, P < 0.001). Left temporoparietal Pi/β-ATP ratios correlated with full scale IQ and verbal IQ (r = −0.82, P = 0.006, r = −0.86, P = 0.003, respectively). In summary, temporoparietal cortical hypometabolism was seen in non-demented Parkinson's disease patients with both 31P-MRS and 18FDG-PET, suggesting that both glycolytic and oxidative pathways are impaired. This dysfunction may reflect either the presence of primary cortical pathology or deafferentation of striato-cortical projections. 31P-MRS and 18FDG-PET may both provide useful predictors of future cognitive impairment in a subset of Parkinson's disease patients who go on to develop dementia.